Method of manufacturing poly-v pulleys from sheet metal

ABSTRACT

The invention presents a manufacturing method of making poly-V pulleys for bending the peripheral end part of the disc-shaped sheet metal blank in concave or convex form, flattening the bent portion so as not to elongate it in the radial direction, increasing the thickness of the peripheral end part of the disc-shaped sheet metal blank, forming the thickened sheet metal blank into a cup form, and forming poly-V grooves at the outer side of the peripheral wall of the cup-shaped blank, and the products thereof. By increasing the thickness of the part corresponding to the peripheral wall of the cup-shaped blank in the stage of the disc-shaped sheet metal blank, the peripheral wall of the cup-shaped blank may be increased in thickness by a simple press machine and at high efficiency.

CROSS-REFERENCE TO RELATED APPLICATION

This application contains subject matter related to application Ser. No.07/646,730, filed Feb. 5, 1991.

TECHNICAL FIELD

The present invention relates to a method of manufacturing poly-Vpulleys from sheet metal from, which way a disc-shaped sheet metalmaterial, and the products thereof.

BACKGROUND ART

Poly-V pulleys are intended to transmit high speed rotation efficiently,and are used in the rotary transmission system of automobiles and othermachines. A conventional example of such poly-V pulleys made from sheetmetal made is disclosed in Layed-Open Japanese Patent Application No.57-88929.

The manufacturing method of making poly-V pulleys from sheet metaldisclosed in this Layed-Open Patent Application comprises the steps offorming a cup-shaped blank by drawing a disc-shaped sheet metalmaterial, drawing the corner part of the boundary area of the bottomwall and peripheral wall of the cup-shaped blank to form the doublefolded ears projecting outside the peripheral wall in an annular shape,folding the outer edge part of the peripheral wall outward to form theears projecting outside the peripheral wall in an annular shape,corrugating the peripheral wall by pressing in the axial direction withthe bottom pattern and top pattern, and flattening the corrugatedportion with the inner pattern and outer pattern by pressing from bothsides to increase the wall thickness of the peripheral wall, and formingby rolling a poly-V groove at the outer side of the peripheral wall ofthe thus thickened cup-shaped blank.

In this manufacturing method, in order to increase the thickness of theperipheral wall of the cup-shaped blank, the disc-shaped sheet metalmaterial is once formed into a cup shape, and its peripheral wall isincreased in thickness, which requires complicated forming machines andtakes a long time to increasing the wall thickness and forming.

It is therefore a primary object of the present invention to present anovel manufacturing method of making poly-V pulleys of sheet metalcapable of increasing the thickness of the peripheral wall of acup-shaped blank efficiently by simple forming equipment, and productsobtained by this manufacturing method.

SUMMARY OF THE INVENTION

To achieve the above object, the invention presents a manufacturingmethod of making poly-V pulleys of sheet metal out of a sheet metalblank formed in a disc shape which comprises a peripheral end foldingstep for folding the peripheral end of the sheet metal blank in concaveor convex form in a specified width range, a peripheral end thickeningstep for increasing the wall thickness by holding and flattening theperipheral end of the sheet metal blank folded in concave or convex formso as to be smaller in diameter than the diameter of the sheet metalblank before being folded in concave or convex form, and a cup-shapedblank forming step for bending the thickened peripheral end in onedirection to form a cup-shaped blank, wherein plural V-grooves areformed at the outer side of the peripheral wall of the thus formedcup-shaped blank.

According to this manufacturing method, since the thickness of theportion corresponding to the peripheral wall of the cup-shaped blank canbe increased, especially in a state of sheet metal blank formed in adisc shape, the conventional complicated thickening and formingequipment is not needed, and the portion corresponding to the peripheralwall of the cup shaped blank may be promptly increased in thickness bysimple pressing equipment for pressing the disc shaped sheet metal blankfrom top and bottom.

When thus thickened the disc-shaped sheet metal blank is formed in a cupshape in the conventional procedure, a cup-shaped blank having athickened peripheral wall is obtained.

In this manner, the manufacturing method of the invention is capable ofincreasing the thickness of the peripheral wall of a cup-shaped blank bysimple equipment and at high efficiency.

The manufacturing method of the invention may, meanwhile, comprise aperipheral wall thickening step, if required, after the cup-shaped blankforming step, for further increasing the thickness of the peripheralwall by bending and forming the peripheral wall of the cup-shaped blank,and holding and compressing the bent portion from inside and outside sothat the upper and lower ends may not extend in the vertical direction.

The sheet metal made poly-V pulleys of the invention are manufacturedthrough the peripheral end thickening step, or through this step and thethickening step, and also include those manufactured by forming anindentation portion in the bottom wall of the cup-shaped blankconcentrically with this cup-shaped blank, those having a bearing placedin this indentation portion, and those having the bottom wall of theindentation portion in the axial center or near the axial center of thepoly-V groove forming region formed in the peripheral wall of thecup-shaped blank.

Other features and effects of the invention will be better understoodand appreciated from the following detailed description of theembodiments of the invention taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to FIG. 1L are explanatory drawings for showing themanufacturing method of making poly-V pulleys of sheet metal accordingto an embodiment of the invention,

FIG. 2 is an explanatory drawing of a peripheral end thickening step,

FIG. 3 is an explanatory drawing of a cup-shaped blank forming step,

FIG. 4 is an explanatory drawing of a corner forming step,

FIG. 5A to FIG. 5C are explanatory drawings of an ear forming step,

FIG. 6A and FIG. 6B are explanatory drawings of a peripheral wallthickening step,

FIG. 7 is an explanatory drawing of a poly-V groove forming step,

FIG. 8 is an explanatory drawing of other poly-V groove forming step,and

FIG. 9 to FIG. 11 are sectional views of products according to theembodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, the invention is described in detailbelow.

FIG. 1A to FIG. 1L illustrate the manufacturing method of making poly-Vpulleys of sheet metal according to an embodiment of the invention. Thismethod comprises a peripheral end folding step, a peripheral endthickening step, a cup-shaped blank forming step, ear forming step, andpoly-V groove forming step. Each step is described in detail below.

(1) Peripheral end folding step

In this step, a metal disc-shaped flat sheet blank (hereinafter calledsheet) 1 shown in FIG. 1A is pressed, and peripheral ends 2 are foldedin a concave or convex form in a specified width range as shown in FIG.1B or FIG. 1C. The method of folding may result in either a V-form asshown in FIG. 1B or a U-form as shown in FIG. 1C, or may it result in aundulated or corrugated form (not shown in the drawing). In thisperipheral end folding step, the initial diameter D1 of the sheet 1becomes slightly shorter to D2. For example, the sheet 1 with D1=138 mmbecomes a sheet 1 of D2=133.9 mm. The thickness t1 of the sheet 1 isunchanged, and if changed, the extent of the change is insignificant.This step is executed as a pretreatment for the peripheral endthickening step.

(2) Peripheral end thickening step

In this step, the thickness of the peripheral ends 2 is increased byflattening the peripheral ends 2 of the sheet 1 of FIG. 1B or FIG. 1C.This step is achieved by pressing the sheet 1 by means of a bottompattern tool 50 and a top pattern tool 51. To carry out this step theedges of the sheet 1 are held so that the diameter D3 of the sheet 1 isachieved which is smaller than the diameter D1 of the sheet 1 shown inFIG. 1A, that is, the sheet 1 before being folded into a concave orconvex form. It is, however, not required that the diameter D3 of thesheet 1 be smaller than the diameter D2 of the sheet 1 shown in FIG. 1Bor FIG. 1C. Therefore, this step is usually carried out of by holdingthe sheet 1 so that the diameter D3 of the sheet 1 after this step maybe equal to the diameter D2 of the sheet 1 shown in FIG. 1B or FIG. 1C,or by holding the edges of the sheet 1 so that it may be slightly largerthan the diameter D2. For achieving the dimensional relationship of thediameters D1, D2, D3, a step 53 in FIG. 2, is provided in the toppattern tool 50 or bottom pattern tool 52 for restricting the limitlessdiameter extent extending is provided, and the inside diameter D of thestep 53 is set in a range of D1>D≧D2. The diameter D3 of the sheet 1after this step is equal to or nearly equal to D, and for example whenthe angle θ of the peripheral ends 2 of the sheet 1 shown in FIG. 1B isset at 35 degrees, D3 is 134.7 mm. Usually, after this step, theperipheral ends 2 of the sheet 1 are roughly flattened, but to beprecise, as shown in FIG. 1D, the surface is visibly corrugated. In thiscase, the corrugated shape is flattened to an invisible level in thesubsequent cup-shape blank forming step. But, of course, the peripheralends 2 may be flattened to such an extent that the corrugated surfacemay not be visible in this step.

The peripheral ends 2 of the sheet 1 are increased in thickness in thisstep, but the thickened portion may also include areas other than theperipheral ends 2. For example, when the peripheral ends 2 arecorrugated as shown in FIG. 1D and the initial thickness t1 of theperipheral ends 2 is 2.6 mm, the thickness of the parts of theperipheral ends 2 of the sheet 1 after this step is 2.75 mm in thethinnest part, and 2.8 to 2.85 mm in the thickest part.

(3) Cup-shaped blank forming step

This is a step of making a cup-shaped blank 3 as shown in FIG. 1E, 1F or1I by bending the thickened peripheral ends 2 of the sheet 1 in onedirection. The cup-shaped blank 3 fabricated in this step possesses aperipheral wall 5 and a bottom wall 6, and the bottom wall 6 may beeither flat as shown in FIG. 1E and FIG. 1F, or bulged out in the middleas shown in FIG. 1I, or although not shown in the drawings, the middlepart may be indented or the bottom may be indented like a flat bowl.

This step may also comprise a bending step for drawing and folding thethickened peripheral ends 2 of the sheet 1 as shown in FIG. 1E, and acorner forming step for forming the curved outer circumference of thecorner 4 at the crossing point of the peripheral wall 5 and bottom wall6 of the cup-shaped blank 3 after the bending step in a right-angle ornearly right-angle shape.

In this case, bending by the drawing step is preferably executed byfolding the peripheral ends (see FIG. 1D) in one direction while holdingthe sheet 1 in FIG. 1D between the bottom pattern tool 54 and the toppattern tool 55 shown in FIG. 3. At this time, by setting the gapbetween the bottom pattern tool 54 and the top pattern tool 55 slightlywider than the dimension of the wall thickness t2 of the peripheral ends2 thickened in the peripheral end thickening step, the thickness t4 ofthe peripheral wall 5 of the resulting cup-shaped blank 3 is slightlygreater than the thickness t2, so that the peripheral wall 5 isincreased in thickness. For example, if the initial wall thickness t2 is2.75 to 2.85 mm as shown above, the thickness t4 of the peripheral wall5 of the cup-shaped blank fabricated in this bending step is 2.75 to 2.8mm in the thinnest part, and 2.9 to 3.0 mm in the thickest part.

At the end of the peripheral end thickening step, if the peripheral ends2 of the sheet 1 are visibly corrugated, the surface may be corrected inthis step so that the corrugation may not be visible, but it is notabsolutely necessary, and it may be corrected gradually in this bendingstep and the subsequent corner forming step.

The corner forming step is preferably executed by holding the cup-shapedblank 3 after the bending step between the bottom pattern tool 56 andthe top pattern tool 57 as shown in FIG. 4. At this time, the lower endof the peripheral wall 5 is defined by the bottom pattern tool 56. Thus,the thickness t5 of the peripheral wall 5 of the cup shaped blank 3 isequal to or slightly greater than the thickness t4, and, at the sametime, the corrugation of the peripheral wall 2 obvious at the end of thebending step is corrected to a high degree, and the thickness t5 becomesuniform at all parts. For example, when the cup-shaped blank 3, afterthe bending step is processed in the corner forming step, the thicknesst5 of the peripheral wall 2 is 3.0 mm in all parts.

(4) Ear forming step

In this step, at the root portion of the peripheral wall 5 correspondingto the thickness range of the bottom wall 6 of the cup-shaped blank 3after the cup-shaped blank forming step as shown in FIG. 1J, a firstannular ear 7 extending outside of the peripheral wall 5 and a secondannular ear 9 extending outside the peripheral wall 5 at the outer edgeof the peripheral wall 5 of the cup-shaped blank 3 are formed byrolling.

The first ear 7 is formed as follows. As shown in FIG. 5A, whilerotating a circular bottom pattern tool 60 by putting the cup-shapedblank 3 thereon, a pointed protrusion 62 formed on an outer pattern tool61 is pressed against the root portion of the peripheral wall 5 of thecup-shaped blank (at the position corresponding to the thickness rangeof the bottom wall 6 of the cup-shaped blank 3), and a V-groove 8 isformed in the root portion of the peripheral wall 5 as if tearing thebottom wall 6 by this protrusion 62 as shown in FIG. 5B and FIG. 5C,while the first ear 7 is formed. Thus, in the forming process of thefirst ear 7, a flow in the material is created in the course ofgradually turning deeply as if tearing the V-groove 8 by the protrusion62, and this material flow is led to both sides of the V-groove 8 in themidst of forming, and the material flow led upward in FIG. 5B extendsoutside of the peripheral wall 5, thereby forming the first ear 7 in anannular shape.

The second ear 9 is formed as follows. As shown in FIG. 5A, whilerotating the circular bottom pattern tool 60 by putting the cup-shapedblank 3 thereon, the peripheral wall 5 of the cup-shaped blank 3 ispressed by the outer pattern tool 61, and the second ear 9 is extendedoutside of the peripheral wall 5 at the outer edge of the peripheralwall 5. The second ear 9 is formed in this way because a flow ofmaterial is created as the peripheral wall 5 is pressed by the circularinner pattern tool 60 and outer pattern tool 61 and this material flowis led to the circumference of the outer edge.

The first ear 7 and the second ear 9 may be formed simultaneously byusing common pattern tools, or separately by using individual patterntools.

After the ear forming step, the thickness t7 of the peripheral wall 5 isreduced, but since the peripheral wall 5 has been preliminarilythickened as stated above, it is possible to prevent the peripheral wall5 from becoming thinner than it was prior to forming the first ear 7 andthe second ear 9.

Thus formed first ear 7 and second ear 9 are made of a single layer.

The V-groove 8 formed in this step is to function as the groove to beengaged with the edge of the poly-V belt together with poly-V groove 10described below. It is desired, meanwhile, to form a relief part to befree from contact with the poly-V belt in the first ear 7. This reliefpart may be easily formed, for example, by building up a bulge-out part62a at the outside of the convex protrusion 62 as shown in FIG. 5A.Similarly, by forming a bulge-out part 62b at the lower side of theouter pattern tool 61, a relief part not contacting with the poly-V beltmay be easily formed in the second ear 9.

This step is effected on the cup-shaped blank 3 not only after thebending step but after the corner forming step in order to obtain thefirst ear 7, that is, in order to have the first ear project outwardlymore in the radial direction, but in the case of the two-piece laminateear formation, as disclosed in the Laid-Open Japanese Patent ApplicationNo. 57-88929, instead of the single layer structure of the first ear 7,the same two-piece laminate ear may be formed the same as in the priorart on the cup-shaped blank 3 formed in the bending step by drawing.

(5) Poly-V groove forming step

This is a step for forming a poly-V groove 10 on the outer surface ofthe peripheral wall 5 of the cup-shaped blank 3.

This step is achieved by forming poly-V groove 10 composed of pluralV-groove groups in the peripheral wall 5 while holding the cup-shapedblank 3 between the circular inner pattern tool 63 and the circularouter pattern tool 64 as shown in FIG. 7 or FIG. 8. Instead of formingthe poly-V groove 10 by one rolling process, it is preferable to form itby plural rolling processes, comprising a preliminary poly-V grooveforming step, and a finishing step for further forming the poly-V grooveof the cup-shaped blank 3 after the preliminary poly-V groove formingstep and finishing the depth and pitch as demanded.

This step may be performed either as shown in FIG. 7, in which a formingplane 63a of the circular inner pattern tool 63 and a forming plane 64aof the circular outer pattern 64 alternately possess a bottom and peak,respectively and are formed to be engaged with each other, or as shownin FIG. 8, in which a forming plane 63a of the circular inner patterntool 63 is flat in the vertical direction, and a forming plane 64a ofthe circular outer pattern tool 64 is formed in an undulated surfacehaving an alternating bottom and peak. According to the method shown inFIG. 7, and as shown in FIG. 1K, a poly-V pulley 12 made of sheet metalpossessing a poly-V groove 10 on the outer surface and having a shellpart 11 undulated on the inner surface is fabricated. According to themethod shown in FIG. 8, and as shown in FIG. 1L, a poly-V pulley 12 madeof sheet metal possessing a poly-V groove 10 on the outer surface andhaving a shell part 11 being straight inside in the vertical directionis fabricated.

This step may be effected either on the cup-shaped blank 3 after thecup-shaped blank forming step including both the bending step and thecorner forming step, or on the cup-shaped blank 3 after the cup-shapedblank forming step consisting only of the bending step.

Besides, between the cup-shaped blank forming step and the ear formingstep, a peripheral wall thickening step may be applied if necessary.

This peripheral wall thickening step is intended to increase thethickness of the peripheral wall by bending and deforming the peripheralwall 5 of the cup-shaped blank 3, and holding and compressing the bentpart from inside and outside while holding so that the upper and lowerends of the peripheral wall 5 may not elongate in the verticaldirection. This step is achieved by, for example, putting the cup-shapedblank 3 having the peripheral wall 5 bent and deformed in a convex,concave or corrugated shape on the circular inner pattern tool 58 asshown in FIG. 6A, abutting the lower end of the peripheral wall 5 of thecup-shaped blank 3 to the lower surface 58a of the circular innerpattern tool 58, compressing the peripheral wall 5 from inside andoutside by this circular inner pattern tool 58 and the outer patterntool 59 to straighten the bending as shown in FIG. 6B, and accordinglyincreasing the thickness t6 of the peripheral wall 5.

In this manufacturing method of making poly-V pulleys of sheet metal,when forming the first ear 7 by one layer only, the diameter of themetal disc-shaped sheet metal blank may be reduced as compared with theprior art of forming the ear by joining two pieces, and the materialcost may be saved, while the weight of the obtained product may bereduced, and furthermore, the axial dimension from one ear 7 to theother ear 9 may be notably shortened advantageously.

FIG. 9 to FIG. 11 show the products of embodiments of the invention.These products are manufactured according to the method describedherein.

The shell part 11 of the poly-V pulley 12 made of sheet metal and shownin FIG. 9 is similar in composition to the shell part 11 explained inFIG. 1K, and poly-V grooves 8, 10 are formed at the outer side, with theinner surface being undulated. Both the first ear 7 and the second ear 9are of a single layer structure. In this construction, an indentationpart 13 is formed concentrically with the shell part 11 in the sideplate composed of the bottom wall 6 of the cup-shaped blank 3 mentionedabove. Such poly-V pulleys 12 are used by winding a poly-V belt on theshell part 11.

The poly-V pulley 12 made of sheet metal and shown in FIG. 10 has abearing 14 fitted in the indentation part 13. This is used by fitting ashaft (not shown) to the bearing 14. Meanwhile, by matching the axialcenter of the bearing 14 with the axial center of the shell part 11,excessive force is hardly applied to the bearing 14 when in use.

The poly-V pulley 12 made of sheet metal and shown in FIG. 11 has thebottom wall 13a of the indentation part 13 positioned in the axialcenter or near the axial center of the forming region of poly-V grooves8, 10 in the shell part 11. In the diagram, the single-dot chain line Xdenotes the axial center line in the forming region, r indicates thedeviation between the center line X and the bottom wall 13a, and thevalue of r is zero when the bottom wall 13a is positioned in the axialcenter of the forming region. Numeral 15 is a boss for shaft mounting.By this poly-V pulley 12 made of sheet metal, at the position of thebottom wall 13a joining with the edge of the annular flange of the boss15, large bending stresses from the poly-V belt do not occur, and thispoint where cracks were often formed is conventionally improved, anddurability enhanced.

Thus, according to the manufacturing method for making poly-V pulleys ofsheet metal by the invention, since the peripheral wall of thecup-shaped blank may be increased in thickness at high efficiency, it isadvantageous for the mass production of poly-V pulleys made of sheetmetal, while the products obtained by this manufacturing method may beused the same as before without any inconvenience.

What is claimed is:
 1. A manufacturing method of making poly-V pulleysfrom disc-shaped sheet metal blanks, comprising the steps of:bending theperipheral end part of the sheet metal blank in a concave or convex formforming a bottom wall and a peripheral wall in a specific width range,increasing the wall thickness of the peripheral wall by holding andflattening the peripheral end part so that the diameter of the foldedblank may be smaller than the diameter of the sheet metal blank beforebeing folded, forming a cup-shaped blank by bending the thus thickenedperipheral wall in one direction, and forming a plurality of V-groovesin the outer side of the peripheral wall of the cup-shaped blank.
 2. Amanufacturing method of making poly-V pulleys according to claim 1,further comprising the steps of: forming the outer circumference of thecorner at the intersection of the peripheral wall and bottom wall atsubstantially a right-angle shape, forming an annular ear extendingoutside of the peripheral wall at the root part of the peripheral wallcorresponding to the thickness range of the bottom wall and an annularear extending outside of the peripheral wall at the outer edge of theperipheral wall.
 3. A manufacturing method of making poly-V pulleysaccording to claim 2, wherein the poly-V groove is formed by rollingwhile holding the peripheral wall of the cup-shaped blank between aninner pattern tool and outer pattern tool.
 4. A manufacturing method ofmaking poly-V pulleys according to claim 2, wherein each pattern toolhas a forming plane with the forming plane of the inner pattern tool andthe forming plane of the outer pattern tool used in forming the poly-Vgrooves having alternately the bottom and peak of the grooves and areformed so as to be engaged with each other, and wherein the cup-shapedblank is held between the inner pattern tool and the outer pattern toolto form the poly-V grooves by rolling.
 5. A manufacturing method ofmaking poly-V pulleys according to claim 2, wherein each pattern toolhas a forming plane with the forming plane of the inner pattern toolbeing flat, and the forming plane of the outer pattern tool being formedas an undulated surface having an alternating bottom and peak, andwherein the cup-shaped blank is held between the inner pattern tool andthe outer pattern tool to form poly-V grooves by rolling.
 6. Amanufacturing method of making poly-V pulleys according to claim 2,further comprising the steps of: forming V-grooves in the root part ofthe peripheral wall as if tearing the bottom wall of the cup-shapedblank, and forming annular ears extending outside of the peripheral wallalong with the rolling process of V-grooves.
 7. A manufacturing methodof making poly-V pulleys according to claim 6, wherein the poly-V grooveis formed by rolling while holding the peripheral wall of the cup-shapedblank between an inner pattern tool and outer pattern tool.
 8. Amanufacturing method of making poly-V pulleys according to claim 6,wherein each pattern tool has a forming plane with the forming plane ofthe inner pattern tool and the forming plane of the outer pattern toolused in forming the poly-V grooves having alternately the bottom andpeak of the grooves and are formed so as to be engaged with each other,and wherein the cup-shaped blank is held between the inner pattern tooland the outer pattern tool to form the poly-V grooves by rolling.
 9. Amanufacturing method of making poly-V pulleys according to claim 6,wherein each tool has a forming plane with the forming plane of theinner pattern tool being flat, and the forming plane of the outerpattern tool being formed as an undulated surface having an alternatingbottom and peak, and wherein the cup-shaped blank is held between theinner pattern tool and the outer pattern tool to form poly-V grooves byrolling.
 10. A manufacturing method of making poly-V pulleys accordingto claim 1, wherein the poly-V grooves are formed by rolling whileholding the peripheral wall of the cup-shaped blank between an innerpattern tool and an outer pattern tool.
 11. A manufacturing method ofmaking poly-V pulleys according to claim 1, wherein each pattern toolhas a forming plane with the forming plane of the inner pattern tool andthe forming plane of the outer pattern tool used in forming the poly-Vgrooves having alternately the bottom and peak of the grooves and areformed so as to be engaged with each other, and wherein the cup-shapedblank is held between the inner pattern tool and the outer pattern toolto form the poly-V grooves by rolling.
 12. A manufacturing method ofmaking poly-V pulleys according to claim 1, wherein each pattern toolhas a forming plane with the forming plane of the inner pattern toolbeing flat, and the forming plane of the outer pattern tool being formedas an undulated surface having an alternating bottom and peak, andwherein the cup-shaped blank is held between the inner pattern tool andthe outer pattern tool to form poly-V grooves by rolling.
 13. Amanufacturing method of making poly-V pulleys according to claim 1,further comprising the steps of: increasing the thickness of theperipheral wall, by bending and deforming the peripheral wall of thecup-shaped blank after the cup-shaped blank forming step, and holdingand compressing the bent portion from inside and outside so that theupper and lower ends of the peripheral wall may not elongate in thevertical direction.